This invention is related to a method for making a retroreflective plate and a pin unit for making said retroreflective plate, and particularly to a utilization of injection molding method for making a plurality of pin units. The pin units are placed on a base and a master negative die having complementary construction to the shape of the pin units is formed by electroforming. The master negative die is then used for making a retroreflective plate with the construction thereof complementary to that of the master negative die. The present invention significantly reduces the manufacture costs and time of retroreflective plates. In addition, the retroreflective plates with different patterns and functions can be made in a more precise and convenient manner.
Retroreflective plates are widely employed for a variety of safety and decorative purposes, and are particularly useful when the need for night time visibility is important under conditions of low ambient light. In most of the retroreflective plates, the light rays impinging upon the front surface of the plates are returned towards the light source in a substantially parallel path. In some other retroreflective plates, the reflected light can be diverged so as to be observed in a wider range by slightly modifying the reflection angle. The retroreflective plates can be used on road warning signs or the light reflector on various vehicles to achieve the decorative and safety purposes.
The conventional method for making the retroreflective plate needs to produce a plurality of metallic pin units (10), as shown in FIG. 1, by precision machining. The pin units (10) are arranged in good order (as shown in FIG. 2) and are clamped together as a bundle by a clamp or other fastening devices. A master negative die (15), as illustrated in FIG. 3, of metallic or other appropriate materials is formed in accordance with the surfaces of the bundle of the pin units (10) by electroforming. The construction of the master negative die (15) is complementary to that of the bundle of the pin units (10). A retroreflective plate (16), as shown in FIG. 4, made of a transparent thermoplastic material or other suitable materials can be formed in cooperation with the master negative die (15). The outer surface (18) of the retroreflective plate (16) is generally a flat plane while the inner side of the retroreflective plate (16) forms a plurality of prisms (17) being complementary to the construction of the master negative die (15). Therefore, the contour and angle of the prisms (17) are substantially the same with those of the pin units (10). Light coming onto the retroreflective plate (16) from the outer surface (18) thereof can be totally reflected back along the parallel direction.
Persons skilled in the art should be familiar with the principle of total reflection by the retroreflective plate (16). Since the refraction coefficient of the retroreflective plate (16) and the refraction coefficient of the ambient air are different from each other, the incident light from the outer surface (18) onto the retroreflective plate (16) can be totally reflected within the prisms (17), and be sent back through the outer surface (18) into the air, as long as the angle between the incident light and the prisms (17) is properly designed and formed.
It has been understood that the above conventional manufacture process involves several disadvantages as discussed hereinafter. As shown in FIG. 1, the pin unit (10) is in the shape of a hexagonal column (or other configuration) with a sharp tip (12) at one end thereof formed by three inclined faces (11). The three inclined faces (11) are substantially in rectangular shape and are substantially perpendicular with one another. It should be noted that the angles among the inclined faces (11) should be very accurate and the surfaces of the inclined faces (11) must be very smooth and even so as to make the retroreflective plate (16) having a plurality of trihedral prisms (17) with satisfactory reflective effect. As can be seen in FIGS. 2 and 4, the shape of the prism (17) of the retroreflective plate (16) is basically identical with the sharp tip (12) of the pin unit (10). In conventional methods for forming the retroreflective plate (16), the pin unit (10) is made of metallic material and is then individually precisely machined by mechanical apparatus. Hence, the conventional methods for producing the retroreflective plate (16) are very time consuming and the manufacture costs thereof are relatively high. In addition, the precision of the above machining process and the homogeneity among the pin units (10) cannot be easily controlled or maintained and thus, an unsatisfactory pin unit may be formed causing an undesired waste of materials and time.
In the conventional process for making the retroreflective plate (16), the pin units (10) are in the shape of a hexagonal column and are tightly arranged in good order, similar to honeycomb, to produce the master negative die (15). Of course, the arrangement of the pin units will not form a honeycomb structure if they are not in the shape of a hexagonal column but other configurations, such as a rectangular column. FIG. 2 illustrates one of the arrangement manners of the pin units (10), wherein a set of pin units (13) and the other set of pin units (14) are rotated 180xc2x0 (60xc2x0 or 300xc2x0) from each other along the longitudinal axis thereof. The master negative die (15) (as shown in FIG. 3) and the retroreflective plate (16) (as shown in FIG. 4) formed by the two sets of pin units (13, 14), respectively, will thus have two sets of prisms (17) in different orientations to reflect the light coming from different directions. However, producing such a retroreflective plate (16) with two sets of prisms (17) in different orientations is not limited to the method mentioned above. A similar retroreflective plate (16) can be made by arranging all the pin units (10) in the same orientation to form two identical master negative dies which are cut in half. One half of a die is rotated 180xc2x0 and combined with the other half of the other die. Nevertheless, there exists in either of the above methods significant difficulty in clamping so many pin units (10) without any relative movement or rotation since there is no connection measure among the pin units (10).
In actual applications, for example serving as the cover for the taillight of vehicle, a light source may be provided within the retroreflective plate (16). Accordingly, a portion of the retroreflective plate (16), as shown in FIG. 4, needs to be flat such that the light cover may not only reflect the light from the exterior environment, but also let the light beam from the light source inside pass through the retroreflective plate (16) and transmit to the exterior environment. One existing method for achieving this function is to form a planar region on each or some of the tips (17xe2x80x2) of the prisms (17) at the inner side of the retroreflective plate (16). Therefore, the tip (12) of the conventional pin unit (10) shown in FIG. 1 needs to be cut off or ground out one by one to make the master negative die (15) with flat regions, so as to further produce the retroreflective plate (16) with the shape of the prisms (17) substantially the same with that of the pin units (10). Alternatively, the manufacturer may grind the tip (17xe2x80x2) of the prism (17) at the inner side of the retroreflective plate (16) after making the retroreflective plate (16) without flat regions. Evidently, both of the above processes are very time consuming resulting in considerably increased production costs, in addition, the precision and quality of the retroreflective plate (16) is difficult to maintain.
It is a primary object of this invention to overcome the defects of the conventional art and to provide a method for making a retroreflective plate. The method of this invention can considerably reduce the production costs and time needed to make the retroreflective plate. In addition, the retroflective plates with different patterns and functions can be made in a more precise and convenient manner.
According to the primary object, this invention provides a method for making a retroreflective plate including the steps of: (a) making a plurality of pin units by injection molding wherein the pin units substantially include a relatively thicker portion and a relatively thinner portion connected to the relatively thicker portion from the bottom thereof along the longitudinal direction of the pin units, the relatively thicker portion having a free end which is away from the relatively thinner portion and forms a tapered portion; (b) placing the pin units on a base, wherein the base has a top surface and a plurality of recessions thereon such that the relatively thinner portion of the pin units can be put into at least some of the recessions for positioning; (c) producing a master negative die with the construction thereof complementary to the tapered portion of the arranged pin units; and (d) using the master negative die to form a retroreflective plate with the construction thereof complementary to that of the master negative die.
One of the primary advantages of this invention resides in that the pin units are produced by injection molding. This injection molding process can not only ensure that the tapered portion of the pin unit has a smooth and even surface and an accurate angle, but also overcome the existing drawback of precisely machining each pin unit individually. Therefore, this invention significantly reduces the time and costs required for making the retroreflective plate.
According to another object of this invention, the pin units are placed into the recessions of the base and thus, relative motion or rotation encountered in conventional process is less likely to occur between the pin units when clamping the same.
According to yet another object of this invention, a mold in a different configuration can be used to inject a plurality of pin units each having a flat end thereon so as to form a master negative die having flat regions to further produce a transparent retroreflective plate.
The special features and characteristics of this invention can be further realized by referring to the appended drawings and explanations of the preferred embodiments.